A variety of optical devices have one or more waveguides positioned on base. The waveguides often guide light signals to other optical components such as modulators, switches, demultiplexers, and light sensors. These waveguides often receive the light signals from an external device and/or guide the light signals to the external optical device. Accordingly, the light signals are often transferred between a waveguide on the optical device and an external optical device such as an optical fiber or laser.
The mode size for the waveguide is often different than mode size of the external optical device. For instance, the mode size of waveguides on planar optical devices is often smaller than the mode size of the optical fibers with which the planar optical device exchanges light signals. The difference in mode size is a source of optical loss that may limit device performance. In order to address this issue, a taper is often added to the waveguide that exchanges light signals with the external optical device. For instance, the waveguide on a planar optical device can be tapered such that the mode size of a light signal carried in the waveguide expands to the mode size of the optical fiber with which the waveguide will exchange light signals.
It is often desirable for these tapers to be inverted such that the bottom of the taper is located below the bottom of an untapered portion of the waveguide. In this configuration, the top of the taper can be flush with the top of the untapered portion of the waveguide to provide the optical device with a continuous and smooth upper surface for the fabrication of other optical components. As a result, fabrication of these tapers typically includes etching downward through a light-transmitting medium to a layer of material that serves as a cladding for the taper. However, this etch generally produces mushrooming at the interface of the light-transmitting medium and underlying the cladding. This mushrooming reduces the ability to control the shape of the taper. As a result, there is a need for an improved taper fabrication techniques and/or structures.